CEDAR APPLE RUST

ONE DRIZZLY SPRING DAY, while wriggling through a thicket in Franklin County, I came face to face with a most horrible sight: an entire cedar tree "eat up" with orange oozes, dripping dew like a thousand noisome noses. But-intrepid biologist that I am-I felt no fear, for I only faced the teliohorns of cedar apple rust (CAR).

Beginning with Cedar: Like other such fungi, CAR demands two hosts to complete its life cycle-cedar trees and apples-and in very precise and particular manners. First, cedar trees support its gruesome galls-roundish, reddish-brown, golfball-sized, and similarly dimpled. With warm spring rains, swollen, horn-like fingers descend from those dimples, dangling increasingly downward and outward with each successive shower-bright yellow-orange tentacles two inches long. During six to eight swelling/drying cycles, a single gall discharges two billion spores, its protuberances finally withering into exhausted, wrinkled threads.

On to the Apples: During the exact same time, nearby apple buds pop open, hopefully (from CAR's selfish viewpoint) providing perfect substrate for spore lodgment and growth. (The fungus prefers several hours of light rain, at fifty to seventy-five degrees Fahrenheit, falling on tender, succulent, four- to eight-day-old leaves or young fruits.) While the wind-blown spores may wander five miles from the "mother" cedar, most apple infections occur within a few hundred feet. There they morph into copper-colored pustules, each with a characteristic chlorotic halo-as many as three hundred spots on a given leaf. Within these putrescent pools, black pycniospores fuse to form hyphae growing toward the opposite leaf surface.

An Etymological Aside: The word heteroecious, combining Greek words for "different" and "house," perfectly describes a rust's life cycle, alternating between two unrelated hosts. (Smuts, the other main type of plant infection, demonstrate an autoeaous or self-housed cycle-e.g., corn smut grows only on corn.) The suffix also helps form ecology, the study of the house where we live, and economics, household management.

Apples, Part Two: Our fungal strand, now massed on the lower leaf surface, erects quarter-inch-long structures ("aecia") directly below the original coppery spots. During the warm, moist weather of late summer-with a temperature of 75°F being optimal-these elongated sacs split into narrow strips and curl backward, releasing chains of airy brown aeciospores, which drift into the cracks and crevices of neighboring cedar twigs. It is in this form that CAR overwinters before growing (through the subsequent year) into the next generation of dimpled galls.

A Brief Review: Teliospores (on cedars), pycniospores (on apples), aeciospores (on apples), and back again. All manifestations of the same creature, but inhabiting different (and specific) parts of different (and specific) plants at different (and specific) times.

Breaking the Cycle: An orchard full of GAR proves quite "galling" to apple growers, defoliating and weakening trees and blemishing their fruits, rendering the latter unmarketable. So why not nip it in the bud-quite literally-by disrupting the heteroecious cycle? This calls for total eradication-every cedar within a spore's throw of apple orchards, plus wild apples and (likewise susceptible) native hawthorns. All must go! But what about that pretty pink crabapple in your mother's yard, or your own perfectly placed foundation planting of ornamental junipers? Just where does the Garden Gestapo stop? It is the veritable teliohorns of a dilemma....

While walking in the woods, though, there is no need to be frightened by an occasional gelatinous tendril. Unless, of course, you share a lot of apple DNA and your leaves are only four to eight days old.

Larry Davenport is a professor of biology atSamford University, Birmingham.

Copyright University of Alabama Press Winter 2005
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